It would be hard to imagine a better time for these two important books to appear. The science of global warming has been making dramatic headlines. NASA scientists recently reported that 2005 was the hottest year on record. Researchers studying the oldest core of Greenland ice yet extracted have also reported that there is more heat-trapping carbon dioxide in the atmosphere now than at any other point in the past 650,000 years. The vast majority of climate scientists agree that if we continue pumping greenhouse gases into the atmosphere the world's temperature will climb significantly, and new computer models project a grim scenario of droughts and rising sea levels. Global warming is a fiendishly complex scientific puzzle, and "The Weather Makers" and "Field Notes From a Catastrophe" help show how the individual pieces fit together into a worrying whole.

It's also a fiendishly complex political puzzle, and there may not be much time to decide how to act. Some leading climate scientists warn that we might be as few as 20 years away from a "tipping point," after which it will be too late to reverse catastrophic change. Yet so far such warnings have not led to much meaningful action. The Bush administration proposes cutting carbon emissions by investing in hybrid cars and other futuristic technologies. Meanwhile, many of the nations that signed the Kyoto Protocols are failing to meet their own targets.

Tim Flannery, a distinguished Australian scientist, and Elizabeth Kolbert, a staff writer for The New Yorker, hope to seize this moment and make the world take global warming seriously. "If humans pursue a business-as-usual course for the first half of this century, I believe the collapse of civilization due to climate change becomes inevitable," Flannery warns. His book may be having an impact already: last October, Australia's environment minister cited Flannery's book when he told a reporter unequivocally that the debate over global warming was over and industrialized nations needed to take urgent action. Still, it's hard to know whether these two passionate, well-researched books will have an enduring effect or will just join a long list of earlier titles on global warming that have not slowed down the greenhouse express. And both books have flaws that may blunt their effectiveness.

While "The Weather Makers" and "Field Notes From a Catastrophe" cover much of the same scientific ground, they are not carbon copies. Flannery, who has written several previous books for a popular audience, takes a long view, offerng an account of the history of earth's shifting climate. Climate change, he makes clear, is itself nothing new, and organisms have long played a role in it. Ever since earth formed some 4.5 billion years ago, heat-trapping gases have kept the atmosphere warm. The planet has simmered and cooled, its changing temperature influenced in part by fluctuating levels of greenhouse gases. Life itself has helped control global warming, both by absorbing greenhouse gases and then by releasing them at death. Sometimes this release has been catastrophic. About 55 million years ago, Flannery writes, a surge of carbon dioxide and methane (another greenhouse gas) flooded the atmosphere, raising the average surface temperature of the earth by 9 to 18 degrees Fahrenheit and causing mass extinctions in what he calls a "vast, natural gas-driven equivalent of a barbecue." Scientists suggest that much of the gas had been stored at the bottom of the sea floor by methane-producing bacteria.

Over the past 50 million years, the planet has been gradually cooling as those greenhouse gases dwindled. Antarctica, once covered by forests and roamed by dinosaurs, grew an ice cap. The earth fell into a cycle of ice ages, in which glaciers expanded and then retreated over tens of thousands of years. The trigger for this cycle was probably earth's wobbly orbit, which changes the amount of sunlight reaching the poles. But greenhouse gases seem to have helped drive the cycle. At the beginning of each ice age, levels of carbon dioxide and methane plunge, and at the end they surge back.

The last ice age ended 13,000 years ago, and by 8,000 years ago the global climate had settled into a comparatively stable lull. This "long summer," as Flannery calls it, may have made civilization possible. Only then did agriculture and cities flourish and spread. Ironically, though, civilization brought with it a new source of greenhouse gases -- ourselves. By burning wood, coal and oil, humans liberated the carbon stored away by other forms of life. Viewed on a geological scale, it's as if a bomb went off.

In "Field Notes From a Catastrophe," Kolbert sets out to see the signs of this change. Kolbert -- whose book first appeared as a series of articles in The New Yorker -- visits researchers drilling ice cores in Greenland in order to study ancient climates. Parts of the Greenland ice cap are melting rapidly, and her tent fills up with water. In Alaska, houses are falling into holes in the collapsing permafrost. In England, Kolbert finds changes that are subtler but no less significant. She meets with biologists who survey the ranges of butterflies each year; they've found that some species are steadily shifting their ranges north as the planet warms. Scientists -- who have observed similar migrations elsewhere -- are concerned that this may cause catastrophic extinctions in coming decades. Animals and plants adapted to living on mountainsides can move only so far uphill before they run out of mountainside. Lowland species may find their migration toward the poles obstructed by oceans, mountains or sprawl.

Kolbert is clearly appalled that even in the face of such overwhelming evidence we keep emitting more greenhouse gases each year. Yet she allows herself only a few terse condemnations. She calls American opposition to the Kyoto Protocols "deeply, even obscenely, self-serving." Mostly, she lets the scientists do the talking. Peter deMenocal, an expert on ancient climates at Columbia University, worries that global warming will destroy modern civilization, just as abrupt climate changes caused earlier civilizations to collapse. "The thing they couldn't prepare for was the same thing that we won't prepare for, because in their case they didn't know about it and because in our case the political system can't listen to it," deMenocal says.

"I have tried to keep the discussion of scientific theory to a minimum," Kolbert writes at the beginning of her book. Yet a greater understanding of the science is exactly what we need right now. Climate science often seems counterintuitive. As a result, self-proclaimed global warming "skeptics" are fond of pointing to individual weather stations where temperatures have gradually dropped over the past few decades. But global warming does not mean uniform warming. It means a rise in the mean global temperature. "Field Notes From a Catastrophe" would have benefited from a deeper exploration of the science of global warming.

Flannery makes a different mistake, sometimes overreaching in his attempt to make an absolutely overwhelming case. For example, he cites the currentslaughter in the Darfur region of Sudan as an example of how desertification, fueled by global warming, drives farmers and nomads into conflict. There's good reason to think that global warming has caused desertification in Sudan, but it's wrong to ignore the role of the Sudanese government's support for the militias (drawn from herding groups) in their attacks against farming villages. Elsewhere Flannery raises the possibility that global warming will carry malaria into heavily populated mountain valleys such as Mexico City. But the link between climate and malaria is nowhere near as solid as he implies. A recent article about global warming and disease, published in the journal Nature, concluded that when it comes to malaria, "a definitive role of long-term climate trends has not been ascertained."

Both Flannery and Kolbert present current global warming as an unnatural evil -- the result of Promethean tinkering with what should be left alone. Flannery calls it "attempted Gaia-cide," a reference to the concept of the entire biosphere united as a single being. But this is not a useful way to think about global warming; it makes no sense to separate ourselves from nature this way. Long before Henry Ford fired up his first Model T, the climate changed drastically many times, and living things often played a major role in those changes. As trees evolved leaves and roots 350 million years ago, they sucked up an estimated 90 percent of the carbon dioxide in the atmosphere. And again, the methane gas that came surging out of the ocean floor 55 million years ago, leading to a spike in global temperature and mass extinctions, was probably produced by bacteria.

We are only the latest species to alter the atmosphere, and until recently we've been as unwitting as the trees and the bacteria. But now, as Kolbert and Flannery demonstrate, we know a little bit better. Their books do not merely satisfy scientific curiosity. Whatever their flaws, with any luck they may help force us to take more responsibility for our collective actions.